1. Field of the Invention
The present invention relates to thin film magnetic heads provided with a magnetoresistive element and to production processes of the same.
2. Description of the Related Art
Current thin film magnetic heads provided with a magnetoresistive element (MR element) can be classified into anisotropic magnetoresistive (AMR) heads utilizing the anisotropic magnetoresistive effect and giant magnetoresistive (GMR) heads utilizing spin-dependent scattering of the conduction electrons. As an example of such GMR heads, U.S. Pat. No. 5,159,513 discloses a spin valve head having a high magnetoresistive effect in a weak external magnetic field.
FIG. 21 illustrates a schematic configuration is a conventional AMR head. The conventional AMR head comprised of a lower shield layer 7 composed of a magnetic alloy such as sendust (Fe--Al--Si) and a lower gap layer 8 formed on the lower shield layer 7. Onto the lower gap layer 8 is laminated an AMR element layer 10. The AMR layer 10 is comprised of a soft magnetic layer 11, a nonmagnetic conductive layer 12 formed on the layer 11, and a ferromagnetic layer (AMR material layer) 13 formed on the layer 12. On both sides of the AMR element layer 10 are formed permanent magnet layers 15, and lead layers 16 in this order.
Onto these layers are formed an upper gap layer 18 and a upper shield layer 19 in this order.
For optimum operations of such an AMR head, two bias magnetic fields are required to apply to the ferromagnetic layer 13 which exhibits the AMR effect.
A first bias magnetic field is to ensure resistance changes of the ferromagnetic layer 13 to respond linearly to a magnetic flux from a magnetic medium. The first bias magnetic field is applied in perpendicular direction (Z direction in FIG. 21) to the film plane of the magnetic medium and in parallel with the plane of the ferromagnetic layer 13. The first bias magnetic filed is generally called as "lateral bias", where a sensing current is passed from the lead layer 16 to the AMR element layer 10 to produce a current magnetic field and thereby to magnetize the soft magnetic layer 11 in the Z direction, and a lateral bias is thus applied onto the ferromagnetic layer 13 in the Z direction by the magnetization of the soft magnetic filed.
The second bias magnetic filed is generally called as "longitudinal bias", which is applied in parallel with the planes of the magnetic medium and the ferromagnetic layer 13 (X direction in FIG. 21). The longitudinal bias magnetic field is to reduce Barkhausen noises generated by a plenty of magnetic domains formed in the ferromagnetic layer 13, in other words, to smooth resistance changes from the magnetic medium to this magnetic flux with less noises.
Reduction of the Barkhausen noises requires the ferromagnetic layer 13 to be put into a single magnetic domain state. Methods of applying longitudinal bias to this end generally include two techniques, i.e., a technique of providing the permanent magnet layers 15, 15 on both sides of the ferromagnetic layer 13 to utilize a leakage flux from the permanent magnet layers 15; and another technique of utilizing an exchange anisotropic magnetic field generated on a contact boundary surface between an antiferromagnetic layer and a ferromagnetic layer.
As a structure of GMR heads utilizing the exchange coupling by an antiferromagnetic layer is known a spin-valve type head illustrated in FIG. 22.
The GMR head illustrated in FIG. 22 differs from the AMR head illustrated in FIG. 21 in that the former comprises a GMR element layer 20 instead of the AMR element layer 10.
The GMR element layer is comprised of a free magnetic layer 22, a nonmagnetic conductive intermediate layer 23, a pinned magnetic layer 24 and an antiferromagnetic layer 25.
In the configuration shown in FIG. 22, bias in the track direction (X direction in FIG. 22) should be applied onto the free magnetic layer 22 by the permanent magnet layers 15, 15 to ensure that the free magnetic layer 22 has the magnetization oriented in the track direction in a single magnetic domain state, and the pinned magnetic layer 24 should have the magnetization oriented in the Z direction in FIG. 22, in a single magnetic domain state by applying bias in the Z direction, i.e., the direction perpendicular to the magnetization of the free magnetic layer 22. In other words, the magnetization of the pinned magnetic layer 24 should not be changed by a flux from a magnetic medium (in the Z direction in FIG. 22), and the magnetization of the free magnetic layer 22 should rotate in the range of 90.+-..theta..degree. with respect to the magnetization of the pinned magnetic layer 24 to give linear responsivity of the magnetoresistive effect.
A comparatively large bias magnetic field is required to fix the magnetization of the pinned magnetic layer 24 in the Z direction in FIG. 22, and the larger is this bias magnetic field, the better is the fixation done. At least a 100-Oe bias magnetic field is required to overcome an antimagnetic field in the Z direction in FIG. 22 and to inhibit the magnetization from rotating or fluctuating by a flux from a magnetic medium. To obtain the bias magnetic field, the configuration illustrated in FIG. 22 utilizes an exchange anisotropic magnetic field generated by providing the antiferromagnetic layer 25 in contact with the pinned magnetic layer 24.
In such a configuration as shown in FIG. 22, the exchange coupling formed by providing the antiferromagnetic layer 25 in contact with the pinned magnetic layer 24 allows the pinned magnetic layer 24 to have the magnetization oriented and fixed in the Z direction. When a leakage magnetic field from a magnetic medium transferring in the Y direction is applied, the electrical resistance of the GMR element layer 20 changes with changes of the magnetization of the free magnetic layer 22, and hence the leakage magnetic field of the magnetic medium can be detected through the electrical resistance changes.
The bias applied to the free magnetic layer 22 is to ensure the linear responsivity and to reduce Barkhausen noises generated due to the formation of a number of magnetic domains, and applied in a similar manner in the longitudinal bias in the AMR head. In the configuration shown in FIG. 22, permanent magnet layers 15, 15 are provided on both sides of the free magnetic layer 22 and a leakage flux from the permanent magnet layers 15, 15 is used as the bias.
During operation of such a thin film magnetic head, the vicinity of an MR element layer such as an AMR element layer or a GMR element layer is known to rise in temperature readily up to about 120.degree. C. due to heat generated through a stationary sensing current. At such a high temperature, the electrical resistance of a ferromagnetic layer changes due to a high sensitivity of the MR element to temperature changes, and hence the reading signals are disturbed. In the GMR elements, the exchange anisotropic magnetic field by the antiferromagnetic layer 25 composed of, for example, FeMn is highly sensitive to changes in temperature, and decreases almost linearly with respect to the temperature and disappears at about 150.degree. C. (blocking temperature: Tb), so that a stable exchange anisotropic magnetic field cannot be obtained.
To solve these problems, conventional thin film magnetic heads provide upper and lower gap layers 8, 18 made of aluminium oxide (Al.sub.2 O.sub.3) with respect to the AMR element layer 10 or the GMR element layer 20 to dissipate the heat gradually through the gap layers 8, 18 to the shield layers 7, 19 to thereby dissipate it to outside.
Demands have been made to enhance the output of thin film magnetic heads, and to this end, a stationary sensing current density applied to the MR element layer should be increased by making the thickness or depth of the MR element thinner.
In conventional thin film magnetic heads, however, when a stationary sensing current density is increased, the heat generated through the stationary sensing current cannot sufficiently be dissipated from the gap layers 8, 18 made of aluminium oxide (Al.sub.2 O.sub.3). The MR element layer is, therefore, deteriorated or cracked or elements in the constitutive layers of the MR element layer transfer therebetween to disturb compositions of the constitutive components of individual layers and thereby to deteriorate the linear responsivity or reduce the suppressing effect of Barkhausen noises. Accordingly, the output cannot be enhanced by simple miniaturization of the MR element or improvement of the current density by means of an increasing stationary sensing current.
This is because the gap layers 8, 18 formed upper and lower sides of the AMR element layer 10 or the GMR element layer 20 are fabricated of an insulation film having a low thermal conductivity such as Al.sub.2 O.sub.3.
TABLE 1 Thermal conductivity Material (W/mK) Al.sub.2 O.sub.3 30 SiO.sub.2 1.4 AlN 260 SiC 100 C (Diamond) 660 BN 57 Mgo 40 SiAlON 33 Si.sub.3 N.sub.4 37
Table 1 demonstrates thermal conductivity of major insulating materials as bulks.
As evident from Table 1, AlN and other insulating materials have higher thermal conductivity than Al.sub.2 O.sub.3 and SiO.sub.2 which have been conventionally used as the gap layers 8, 18.
Among these materials, AlN is the most preferable insulating material as the gap layers 8, 18 for its high crystallinity and high thermal conductivity.
The present inventors made intensive investigations and experiments on materials of the gap layers, and assumed that the use of aluminium nitride (AlN) having a higher thermal conductivity than aluminium oxide (Al.sub.2 O.sub.3) conventionally used can dissipate the heat generated through a stationary sensing current with a high efficiency.
Such an MR head is produced by a photolithography process comprising photoresist application, exposure, development with a strong alkali solution and rinsing with water. The aluminium nitride (AlN) when constituting the gap layers is highly dissolved in a strong alkali solution which may possibly invite short circuit upon passing of a sensing current is passed. An aluminium nitride film, which readily reacts with water and forms a compound to be dissolved, is readily dissolved in the rinsing process of the manufacture of the MR head or by moisture in air after its manufacture. The reliability of such a head is hence deteriorated.
AlN films used as the gap layers 8, 18 are formed by sputtering or vapor deposition.
As sputtering apparatus are used known apparatus such as an RF sputtering apparatus or a magnetron sputtering apparatus.
According to these sputtering apparatus, a substrate and a target opposed to each other, and the target is mounted on an electrode unit. By applying radio frequencies to the electrode unit from a radio frequency electric source (RF electric source), the target is sputtered to thereby form an AlN film on the substrate.
When power is supplied only to the target upon sputtering as in this sputtering apparatus, however, the AlN film formed on the substrate may readily have pinholes, and its thermal conductivity to serve as the gap layers 8, 18 is decreased its corrosion resistance against an alkali aqueous solution is deteriorated.
The gap layers 8, 18 should be insoluble in an alkali aqueous solution as the alkali aqueous solution is used as a developer in the resist patterning.